1. Field of the Invention
The present invention relates to the field of determining the size, speed, and other parameters of particles, droplets, bubbles, or the like using laser light scattering.
2. Art Background
The measurement of particles, aerosols, liquid drops, bubbles and the like associated with industrial processes, atmospheric monitoring, combustion processes, agricultural applications of chemicals, cavitation studies, and the like has long been of importance. There have been a number of techniques developed that employ laser light scattering to determine the size of particles, drops, bubbles, or the like (hereinafter collectively referred to as "particles"). These techniques utilize one or more of a number of physical phenomena associated with the light scattering to obtain a measurable quantity that may be related to the particle size. The phenomena include the amplitude or intensity, the angular distribution, and the phase shift of the scattered light. Laser light extinction may also be used with other parameters to obtain additional information on the particles. Systems using the phase shift of the scattered light have been described by the inventor, W. D. Bachalo, in articles entitled, "Method for Measuring the Size and Velocity of Spheres by Dual-Beam Light-Scatter Interferometry", Applied Optics, Vol. 19, Feb. 1, 1980; "Phase/Doppler Spray Analyzer for Simultaneous Measurements of Drop Size and Velocity Distributions", and U.S. Pat. No. 4,540,283. Methods using the angular distribution of the scattered light have been described by J. Swithenbank, J. M. Beer, D. S. Taylor, D. Abbott, and G. C. McCreath, "Laser Diagnostic Technique for the Measurement of Droplet and Particle Size Distribution", Progress in Astronautics and Aeronautics, Vol. 53, ed., B. T. inn, 1977; and, E. D. Hirleman and S. Wittig, "In Situ Optical Measurement of Automobile Exhaust Gas Particulate Size Distributions: Regular Fuel and Methanol Mixtures", 16th Symposium (International) on Combustion, MIT, August 1976.
In the present disclosure, the system described utilizes the detection of the amplitude (intensity) of the light scattered by particles to obtain a measurement of their size and speed. The light scattered may be related to the particle size using the well-known Mie theory if the particles are homogeneous and spherical. Calibration with particles of known size may also be used to obtain the functional relationship between the particle size and the scattered light intensity received over a fixed solid angle. A significant difficulty arises when using laser beams with Gaussian (or other nonuniform) intensity profiles. The problem with detecting the peak value of the signal obtained from the scattered light is that this peak value is not only dependent upon the particle size, but also its trajectory through the measurement volume. Since the particle trajectories are random, an uncertainty in the measurement that must be resolved. When designing an in-situ nonintrusive device, this problem places constraints on the implementation of the technique in the field.
At least two viable methods have been proposed to deal with the problem of the Gaussian beam intensity distribution. Holve, D. J., and Self, S., "Optical Particle Sizing for In Situ Measurements", Journal of Applied Optics, Vol 18, No. 10, May 1979, pp. 1646-1652, utilized an inversion technique somewhat analogous to methods used in Computer Aided Tomography (CAT) systems. The numerical inversion scheme is used to unfold the dependence of the signals produced by light scattered by particles traversing the sample volume, formed by the laser beam and receiver optics, on random trajectories. A calibration procedure utilizing monodispersed particles of known size is used to define the sample volume and signal amplitude with respect to the particle size.
The second method for removing the ambiguity associated with the Gaussian beam intensity has been described by the inventor, W. D. Bachalo, in U.S. Pat. No. 4,329,054 which was issued on May 11, 1982. Subsequent disclosures of similar approaches have been described by R. J. Adrian in U.S. Pat. No. 4,387,993, issued June 14, 1983; by Apostolos Goulas, et al., in U.S. Pat. No. 4,348,111, issued Sept. 7, 1982; and R. A. Knollenberg in U.S. Pat. No. 4,636,075, issued Jan. 13, 1987. In each case, two concentric or coaxial beams are used having different wavelengths or polarizations. A beam having one wavelength or polarization is focused to a smaller diameter and directed to the center of a larger beam. In this way, the central uniform intensity of the larger beam may be identified. Only particles passing through the central portion of the larger beam will also produce signals on the small beam. When a signal is received from the small beam, the peak amplitude of the signal from the large beam is read and used to obtain the particle size. The method as described by Bachalo has the disadvantage of requiring a relatively large beam diameter ratio (5:1 to 7:1) between the small (pointer) and large (data) beams. A large beam diameter ratio is necessary to ensure that the incident intensity upon the particle from the large beam is known with sufficient accuracy. This requirement acts as a constraint on the upper limit of particle number densities (particle/cc) in which the system will operate satisfactorily.
Nonetheless, instruments based upon this concept have been developed by Hess and Spinoza (see U.S. Pat. No. 4,537,507, issued Aug. 27, 1985), and by Yeoman, M. L., Azzopardi, B. J., White, H. J., Bates, C. J., and Roberts, P. J., "Eng. Appl. of Laser Velocimetry", Winter Annual Meeting ASME, 1982. Upon careful calibration, the instruments were found to perform satisfactorily. As discussed by Bachalo, the method may be combined with the laser Doppler velocimeter to obtain simultaneous particle size and velocity measurements.
In the cases cited, the requirement for the rather large beam diameter ratios limits the application of the system to rather dilute particle fields. The method of Knollenberg which uses an elongated beam shape, overcomes this problem. However, the optical depth of field of the receiver and the need to measure particles on random trajectories also limits the application of the method.
The present invention discloses a means for significantly improving the above-mentioned technique to remove the serious limitation in high number density particle fields, presented by the need for large beam diameter ratios, allow the simultaneous measurement of particle size, speed, and the sample volume cross-section. A mathematical formulation is given to determine where each particle passed through the Gaussian beam intensity profile and hence, to determine the incident intensity upon the particle. In addition, determination of the individual particle trajectories will allow the measurement of the sample volume diameter for each particle size class. Finally, the particle trajectory through the sample volume along with the transit time will be demonstrated as a means for measuring the speed of the particle. The method has the significant advantage of requiring a beam diameter ratio of only two to three.